50 FORMATION OF THE LAYERS. 



In the embryo from which the sections PL IV. fig. 8 a, 8 h, 

 8 c were taken, the mesoblast had, in most parts, not yet become 

 separated from the hypoblast. It still formed with this a con- 

 tinuous layer, though the mesoblast cells were distinguish- 

 able by their shape from the hypoblast. In only one section 

 (b) was any part of the mesoblast quite separated from the 

 hypoblast. 



In the hindermost part of the embryo the mesoblast is at its 

 maximum, and forms, on each side, a continuous sheet extending 

 from the median line to the periphery (fig. 8 a). The rounder 

 form of the mesoblast cells renders the line of junction between 

 the layer constituted by them and the hypoblast fairly distinct ; 

 but towards the periphery, where the hypoblast cells have the 

 same rounded form as the mesoblast, the fusion between the 

 two layers is nearly complete. 



In an anterior section the mesoblast is only present as a cap 

 on both sides of the medullary groove, and as a mass of cells 

 at the periphery of the section (fig. 8 b) ; but no continuous layer 

 of it is present. In the foremost of the three sections (fig. 8 c) 

 the mesoblast can scarcely be said to have become in any 

 way separated from the hypoblast except at the summit of the 

 medullary folds (m). 



From these and similar sections it may be certainly con- 

 cluded, that the mesoblast becomes first separated from the 

 hypoblast as a distinct layer in the posterior region of the em- 

 bryo, and only at a later period in the region of the head. 



In an embryo but slightly more developed than B, the 

 formation of the layer is quite completed in the region of the 

 embryo. To this embryo I now pass on. 



In the non-embryonic parts of the blastoderm no fresh fea- 

 tures of interest have appeared. It still consists of two layers. 

 The epiblast is composed of flattened cells, and the lower layer 

 of a network of more rounded cells, elongated in a lateral 

 direction. The growth of the blastoderm has continued to be 

 very rapid. 



In the region of the embryo (PI. IV. fig. 9) more important 

 changes have occurred. The epiblast still remains as a single 

 row of columnar cells. The hypoblast is no longer fused with 

 the mesoblast, and forms a distinct dorsal wall for the alimentary 



